The invention is in the field of dual voltage power supplies, and input filters for suppressing electrical transients and reducing electromagnetic radiation (radio interference).
Power supplies operating from a-c current are desirably provided with input filters. For example, U.S. Pat. No. 4,188,661 to Bower and Kohler discloses an electronic ballast circuit for operating fluorescent lamps from a source of a-c electrical power; an input filter has two windings on a common core. U.S. Pat. No. 4,109,307 to Knoll discloses a lamp ballast circuit having an input filter with two windings on separate cores. FIG. 1 of the present drawing illustrates a prior-art dual voltage power supply having an input filter connected ahead of a voltage selection switch. In certain devices, for example movie projectors having an arc discharge lamp operated by a dual voltage power supply, it is desirable to locate the input filter in the power supply unit and electrically after the voltage selection switch, thus requiring two input filters, i.e. one in the 120 volt input line and another in the 240 volt input line.